VERY IMPORTANT!! COMPUTER PROBLEM!!?

[R.S.B]

Hi I am a student currently in my Junior year of high school and i mainly use my computer for regular teenager stuff like facebook and twitter, but i also use it for school...typing papers, doing research, etc and recently my computer has been messing up very badly and any help would be very appreciated!

Okay so i have a Gateway Sx2800-07 that is running a Intel Core 2 Quad proccesor Q8300 with a Intel GMA X4500 integrated video card, with 4gb of ram and 640 gb of memory on the harddrive

http://www.amazon.com/Gateway-SX2800-07-...roduct_top

but anyways the computer works fine for like 10-15 minutes, but then things will stop loading and the start menu will stop working and i cant click on anything or do anything....also it will shut down forever! these problems have been happening every single day now all the time, please help me! My finals are here and i need to do major research projects for my classes and all the help provided would be very much appreciated

Thank You